The Topaz 123
plane,
almost perfect, and it cleaves so easily that a cut topaz, if dropped,
might be easily cracked or broken. The crystallisation of topaz js
imperfect; structure, columnar; lustre, vitreous; streak, white. Topaz
is either transparent or translucent; the colours of topaz including
wine, amber, honey, and straw-yellow, pale blue to pale green of many
shades, greyish, reddish, and white. Kolled pebbles of limpid
colourless topaz are called by Brazilians " pingas d'agoa," and by the French, " gouttes d'eau," both
meaning drops of water. The coloured varieties show marked
pleochroism. The fracture of this mineral is conchoidal and uneven.
True
topaz is a silicate of alumina, containing hydroxl and fluorine;
hardness, 8; specific gravity, 3.4 to 3.6. Being three and one half
times as heavy as water, topaz can be readily distinguished from other
stones resembling it by those accustomed to handling them. Topaz cannot
be fused on charcoal before the blowpipe, but it is partially
decomposed by sulphuric acid. Its hardness enables it to take a high
polish, and the colourless variety has been cut in brilliant or rose
form so as to resemble the diamond, for which it might readily pass in
day-light. However, it is but weakly doubly
